The environmental and economical problems related with diesel fuel are well known, and efforts to substitute a percentage of it with biofuels are underway (European Directive 30/2003 about the necessity of using biofuels as substitutes for diesel fuel).
A generally accepted substitute of diesel fuel is a mixture of methyl esters of fatty acids, produced from the chemical treatment of lipids and mainly vegetable oils. This mixture of methyl esters is called “Biodiesel” defining and confining the general meaning of this term. The European standard EN14214/2003 which defines the technical requirements of Biodiesel refers exclusively to fatty acids methyl esters and to no other possible substitute.
The European Directive 30/2003 defines that vegetable oils that are compatible with diesel engines, and their exhausts are compliant with the requirements of legislation are considered as Biofuels. So far, no one proved indisputably that vegetable oils have the ability to offer high quality biofuels, complying with the technical requirements of the European standards EN14214/2003 for Biodiesel and EN 590/1999 for Diesel fuel. Therefore the question arises what the impediment was of a scientific thorough research of this subject.
The answer mainly lies in the Cetane Number which is a measure of the flammability and quality of diesel fuel, as the octane number is a figure of merit for gasoline burned in spark ignition engines.
It is well known that:                1. Current European Diesel fuel has a Cetane Number of about 52.        2. Vegetable oils have Cetane Number in the range of 40.        3. The lower acceptable limit of Cetane Number for Biodiesel is 51.        
So the experts concluded that vegetable oils are not the proper substitutes for diesel fuel since they will devaluate its quality. Consequently vegetable oils should be chemically transformed to Fatty Acids Methyl Esters acquiring characteristics of an acceptable fuel, regardless of the product cost.
Other factors that prevented the use of vegetable oils as biofuels, were considered the possible problems with engine cold start and filter plugging which might occur at temperatures below 0° C. These problems are improved with the transformation of the oils to methyl esters without being eliminated completely.
All the above were some of the reasons for exception of vegetable oils to be used as substitutes of diesel fuels, leading to the unique solution of the Fatty Acid Methyl Esters, despite the fact of their serious disadvantages. Since the European Directive 30/2003 promulgated the compulsory use of FAME, these disadvantages are either understated or silenced, but they do exist and are mainly the following:                In the mixture of Biodiesel, which is produced from the esterification of fatty acids with methanol, there is a quantity of fatty acids, an inevitable result of the reversible nature of the chemical reaction. The existence of these acids can not be eliminated by using excess of methanol and repeating the procedure several times, due to cost constraints and to the fact that the final price should be close to the price of diesel fuel.        
There are reservations of automotive manufacturers about metal wear in the engines when Biodiesel is used being mixed with diesel fuel, because of the existence of these acids. During the first step of the chemical process for Biodiesel production (hydrolysis) glycerin is produced as byproduct at 10 to 11 percent by volume of Biodiesel. This glycerin has to be sold in the market and used. Currently the market is saturated due to overproduction from the continuously built factories of Biodiesel. Hence the price of Biodiesel is very close to the price of diesel fuel and not attractive to consumers.